Toronto is really cool; so is London. As for the US, my favorite overnights as a flight attendant have been: St Louis, Atlanta (Phatlanta), Kansas City, and St Paul. But, the best city, without contest, HAS to be NYC. No contest. :D
I'd have to say London...at least, from what little time I've spent there. I really hope it's great, anyway, since I'll be living there for at least the next year of my life. I also dig Charlestown, Virginia. Quaint little city, nice downtown with good Italian food.